1. Field of the Invention
This invention relates to a silica-modified silicoaluminophosphate catalyst, a method for preparing the same, and a process for producing methylamines or lower olefins through a methanol conversion reaction using the same. More particularly, the invention relates to a silica-modified silicoaluminophosphate catalyst which is useful for various catalytic reactions, including production of methylamines through a reaction of methanol with ammonia or monomethylamine, production of dimethylamine through a disproportionation reaction of monomethylamine, and production of lower olefins from methanol, and a method for preparing such catalysts, as well as a process for producing methylamines or lower olefins using such catalysts.
Methylamines, specifically dimethylamine, are important materials for production of solvents, typically dimethylformamide, rubber products, pharmaceuticals, surfactants and others. Lower olefins are key materials in the chemical industry.
2. Description of the Related Art
Using methanol which is obtained from natural gas on a large scale at a moderate cost, many efforts have so far been made to convert it to useful chemicals, for example, olefins, ethylene glycol, ethanol, etc. However, these have not yet been successfully conducted on a commercial scale. Important, commercially available methanol derivatives involve formaldehyde and methylamines. It has already been known that methylamines and olefins are produced from methanol by using silicoaluminophosphate molecular sieves as a catalyst. Methylamines have generally been produced from methanol and ammonia at a temperature around 400.degree. C. using a solid catalyst such as silica-alumina. It has also been known that methylamines can be produced from methanol and other methylamines, or through disproportionation of monomethylamine. In these processes for producing methylamines, however, the main product is trimethylamine for which there is little demand in the marketplace. Thus, a process for producing dimethylamine in a higher selectivity has been desired, because of its biased demand among three methylamines.
In recent years, a number of methods for producing methylamines using zeolite catalysts have been proposed, which are more advantageous as compared with the silica-alumina catalysts used so far. For example, there are methods using zeolites, such as zeolite A (JP 56-69846A); FU-1 (JP 54-148708A); ZSM-5 (U.S. Pat. No. 4,082,805); ferrierite and erionite (JP 59-113747A); ZK-5, RHO, chabazite and erionite (JP 61-254256A); and mordenite (JP 56-46846A, JP 58-49340A, JP 59-210050A and JP 59-227841A). Also, a method for producing monomethylamine and the like in an amount exceeding the thermodynamic equilibrium proportion by using a silicoaluminophosphate has been proposed (JP 2-734A).
Meanwhile, ethylene, propylene and other important olefins can be derived from methanol by using zeolites and silicoalumino-phosphates as a catalyst, and a number of methods have been proposed after active studies. Among them, a silicoaluminophosphate catalyst, for example, SAPO-34 catalyst developed by UOP (JP 59-84829A), has been well known from its excellent performances. However, SAPO-34 has small pore diameters of approximately 0.4 nm, and necessitates a high reaction temperature of 400 to 450.degree. C., thereby being suffered from coking, with a short catalyst life of several hours. When the silicoaluminophosphates are prepared according to the conventional methods disclosed in literatures, for example, JP 59-35018A, JP 60-251122A and JP 60-260420A, the products are not free from some disadvantages, such as the long crystallizing period of time, a large amount of impurities, and a low degree of crystallization due to the remaining amorphous components. Furthermore, the silicoaluminophosphates thus prepared have insufficient performances as a methylamine catalyst, in respect to methanol conversion ratio and dimethylamine selectivity, and also as a lower olefin catalyst, in respect to activity, selectivity and catalyst life.